Figure 10.

Proposed roles of SE in Sgs1–Top3–Rmi1 function. (A) SE-dependent D-loop unwinding in the SDSA recombination pathway. After strand invasion and elongation, the nascent strand (red) is displaced by the combined activities of Top3, Rmi1, Sgs-SE and Sgs1 helicase (T, R, SE and H, respectively). Shown in the bracket are intermediate steps: the displaced parental strand is rewound by SE, which anneals the blue parental strands (i) and displaces the red nascent strand (ii) whereas Top3–Rmi1 interlinks the parental strands (iii). Repeating this cycle ‘n' times leads to strand displacement. (B) SE mediates double HJ branch migration. The circular inset presents an example where the red DNA strand of the right-hand HJ is annealed back to its parental complement (in the direction of the dotted arrow). As in branch migration, this displaces the blue strand in the upper heteroduplex so that it can be re-annealed to the lower duplex (in this case by SE), however the DNA between two HJs is topologically constrained. Shown in the bracket is an example of the topological stress encountered on the bottom duplex (i) where the red strand must pass through the blue strand through Top3–Rmi1 activity (ii) yielding one unlinking of the bottom duplex (iii). Not shown is the rewinding of the lower blue strands or reciprocal reactions on the upper duplex. Repeating these cycles of SE- and Top3–Rmi1-mediated strand passage ‘n' times leads to displacement of the heteroduplex between the HJs and formation of a double hemi-catenane. Resolution of the hemi-catenane is promoted by the strand annealing activity of SE and catalysed by Top3–Rmi1 to yield non-crossover products.